The acquisition of data and subsequent generation of computer models for real-world objects is of interest in many industries, for applications including architecture, physical plant design, entertainment applications (e.g., in movies and games), surveying, manufacturing quality control, medical imaging, and construction, as well as cartography and geography applications. In order to obtain accurate models of an object, as well as the area in which that object exists in the real world, it is necessary to take accurate measurements or samplings of surfaces that make up the object and any elements of the surrounding area. Historically, this sampling was carried out by surveyors, photogrammetrists, or technicians using techniques that provided samples at the rate of tens or hundreds per hour at most. Since the amount of data was relatively small, the data was easily dealt with in standard, off-the-shelf CAD programs or other modeling software.
Recent advances in scanning technology, such as technologies utilizing LIDAR scanning, have resulted in the ability to collect billions of point samples on physical surfaces, over large areas, in a matter of hours. In a LIDAR process, a laser beam scans across a view that encompasses the structure of interest. The scanning device measures a large number of points that lie on surfaces visible in the scene. Each scan point has a measured location in 3D space, to within some measurement error, that typically is recorded relative to a point (x,y,z) in the local coordinate system of the scanner. The resulting collection of points is often referred to as one or more point clouds, where each point cloud can include points that lie on many different surfaces in the scanned view. LIDAR systems are described, for example, in U.S. Pat. No. 5,988,862, filed Apr. 24, 1996, entitled “INTEGRATED SYSTEM FOR QUICKLY AND ACCURATELY IMAGING AND MODELING THREE DIMENSIONAL OBJECTS,” which is hereby incorporated herein by reference.
An exemplary surveying system 100 shown in FIG. 1 utilizes a Field Digital Vision (FDV) module 102 that includes a scanning device for scanning an object 104, such as a building of a piece of machinery. The scanning device also can sense the position in three-dimensional space of selected points on the surface of the object 104. The FDV module 102 generates a point cloud 106 that represents the detected positions of the selected points. The point cloud 106 also can represent other attributes of the detected positions, such as reflectivity, surface color, and texture, where desired.
A control and processing station 108 interacts with the FDV 102 to provide control and targeting functions for the scanning sensor. In addition, the processing and control station 108 can utilize software to analyze groups of points in the point cloud 106 to generate a model of the object of interest 104. A user interface 116 allows a user to interact with the system, such as to view a two-dimensional (2D) representation of the three-dimensional (3D) point cloud, or to select a portion of that object to be viewed in higher detail as discussed elsewhere herein. The processing station can include any appropriate components, such as standard computer and/or processing components. The processing station also can have computer code in resident memory, on a local hard drive, or in a removable drive or other memory device, which can be programmed to the processing station or obtained from a computer program product such as a CD-ROM or download signal. The computer code can include instructions for interacting with the FDV and/or a user, and can include instructions for undertaking and completing any modeling and/or scanning process discussed, described, or suggested herein.
The FDV 102 can include an optical transceiver 110 capable of scanning points of the object 104, and that generates a data signal that precisely represents the position in 3D space of each scanned point. The data signal for the groups of scanned points can collectively constitute the point cloud 106. In addition, a video system 112 can be provided, which in one embodiment includes both wide angle and narrow angle CCD cameras. The wide angle CCD camera can acquire a video image of the object 104 and provides to the control and processing station 108, through a control/interface module 114, a signal that represents the acquired video image.
The acquired video image can be displayed to a user through a user interface 116 of the control and processing station 108. Through the user interface 116, the user can select a portion of the image containing an object to be scanned. In response to user input, the control and processing station can provide a scanning control signal to the transceiver 110 for controlling the portion of the surface of the object that should be scanned by the transceiver. More particularly, the scanning control signal can be used to control an accurate and repeatable beam steering mechanism that steers a beam or pulse of the transceiver 110. The narrow angle CCD camera of the video system 112 can capture the intensity returned from each scan impingement point, along with any desired texture and color information, and can provide this captured information to the control and processing station 108. The control and processing station can include a data processing system (e.g., a notebook computer or a graphics workstation) having special purpose software that, when executed, instructs the data processing system to perform the FDV 102 control and targeting functions, and also to perform the model generation functions discussed elsewhere herein. Once the object has been scanned and the data transferred to the control and processing station, the data and/or instructions relating to the data can be displayed to the user.
FIG. 2 shows a block diagram of an optical transceiver 200 of the FDV of the prior art. The optical transceiver 200 transmits an optical pulse to a spot on an object (or structure) being scanned, and receives back an optical pulse reflected from the object. Given the constant speed of light, the optical transceiver calibrates the distance to the spot on the target. A laser 202 in this example is used to generate an optical pulse, which typically lasts less than 250 psec, in response to a command provided from a laser controller 204. The laser 202 produces the pulse, at a wavelength such as about 532 nm, within about 100–300 microseconds after receiving a command signal. The command signal emanates from a digital signal processor that provides central control of real time events. The time delay is a function of variables such as laser age, recent laser history, and environmental/operating conditions. Power and control signals can be provided to the transceiver from a set of scanning control and power components 220. Another set of components 222 can be used that includes additional power electronics, as well as interface components for interfacing with a user, other internal or external devices, and/or processing equipment.
The output of the laser 202 is transmitted through a beam expander 206 that is focused to adjust the size of a light spot that will eventually impinge upon a point on the object being scanned. The focused optical pulse then is transmitted through a duplexer 208, which is an optical system for aligning the outgoing optical path with the incoming optical path. The duplexer 208 directs a significant first portion of the light energy of the outgoing optical pulse to a spot on the object via a scanner. A second but much smaller portion of the light energy of the outgoing optical pulse is directed to a receiver telescope 212. The portion of the outgoing optical pulse that propagates to the object impinges on a spot on the object, and some of the energy of the optical pulse is reflected off the object in a direction back to the duplexer 208. The scanner (not shown) typically includes a beam steering unit, or beam deflection unit, that utilizes one or more mirrors or other optical elements for directing the output pulse. In order to direct the output pulse to scan in two dimensions, a pair of mirrors can be used to direct the pulse along two linear axes. This can include a first mirror device and a second mirror device, each capable of rotating relative to a rotational axis. In one example, a first mirror rotates parallel to a rotational axis of the beam steering unit. A drive motor functions to rotate the second mirror about a second rotational axis, which typically is orthogonal to the rotational axis of the beam steering unit. The rotation along the two axes allows for direction of the beam along a two-dimensional path, such as a raster pattern. Light reflected from the object can be received by the scanner and directed by the first and second mirror devices to the optical transceiver 200.
The returning optical pulse is directed by the duplexer 208 to the receiver telescope 212, which focuses the received energy onto a detector 214. The detector 214 converts the received optical pulse energy into electrical energy. The output of the detector is a series of electrical pulses, the first (generated by the detector in response to the small portion of the transmitted pulse not directed toward the object) occurring at a short fixed time (i.e., fixed by the length of the optical path through the beam expander, duplexer, and receiver telescope) and the second occurring as light energy returns from the object. Both the second, small portion of the transmitted pulse and the return optical pulse reflected from the spot on the object are provided to the timing circuit 216, which calculates the time of flight to the spot on the object. The range to the spot on the object can then be readily calculated from the calculated time of flight.
In order to allow for rotation of the beam deflection unit(s) in the scanner of this example, it is necessary to have a rotary connection allowing at least a portion of each beam deflection unit, such as the first and/or second mirror devices, to rotate relative to the rest of the FDV. Surveying instrumentation can have any of a number of points that require precise rotary components to deliver high angular precision, such as for deflecting light beams and/or adjusting viewing optics. If the rotary portions are instrumented, for example, both information and power may need to flow through this rotary connection. Existing survey instrumentation typically solves this problem through use of electromechanical slip rings. Electro-mechanical slip rings consist of one or more rings and a number of brushes, both the ring(s) and brushes being made of conductive material. The rings or brushes are connected to the stationary unit, with the other part being connected to a rotary portion. An electrical current is applied from the stationary side, such that current passes through the mechanical contact between the ring and brushes. This mechanical contact can be undesirable, as a small dust particle or mechanical imperfection can lead to occasional breaks in the mechanical contact and thus a break in current flow. Breaks in current can lead to increased levels of noise in the system. Another problem is that these mechanical connections have the potential for sparking, which can be a severe detriment in explosive environments. These mechanical connections also experience frictional effects, which can affect the angular precision of the rotation.